An animal can be said to show territorial behaviour when it has some attachment to a site (or occasionally to a moving object) and when it is aggressive towards other animals which approach that place. The resulting territory, around the site of attachment, has been defined in a variety of ways. Pitelka (1959) and Schoener (1968) emphasised the owner's exclusive use of an area, usually with defined boundaries, whereas Davies (1978) recognised territories wherever interactions between individual animals led to their being spaced apart more than would be expected from random settlement. Between these two extremes, I prefer the simple definition of territory as ‘a defended area’ (Noble, 1939; Nice, 1941). This embodies the essential features of a special place, around which there is aggressive defence, without implying particular features such as exclusive use or rigid boundaries, or particular consequences such as spacing out of the individuals, which may occur in many but not necessarily in all cases.

Territorial behaviour raises some interesting questions. Since aggressive defence of an area requires the expenditure of time and effort, there should be some corresponding benefit to the owner's fitness which outweighs the cost of territoriality (Davies, 1978). The spacing effect of territorial aggression would be expected to influence the dispersion pattern and density of populations. These possibilities are of particular interest in the shelduck since territorial defence, particularly by both members of the pair, is not common among ducks.

Shelduck territories

Towards the end of winter, from February to April, shelduck pairs detach themselves from the winter flock and scatter widely over muddy shores, or freshwater pools and creeks near the coast.

On reaching adulthood the young animal faces the problem of settling into the patch of habitat which may be its home for the rest of its life. The selection of an area is crucial to the individual's genetic fitness since it may affect both survival and breeding success and thus the total number of progeny produced in the animal's lifetime. Many individuals settle preferentially in their natal area, but others emigrate and face a choice between different regions for settlement. Selection of an area may depend mainly on assessment of the resources available but should also take into account the degree of competition for them. The response of individuals to increasing competition may limit the total number of young which settle in a local population and this may be important in the limitation of density.

In many territorial species, recruitment occurs in two stages, firstly to the non-territorial flock and then from there to the attainment of a breeding territory, sometimes several years later (Charles, 1972). In this chapter I will describe the return of young adult shelducks to the flock, the factors affecting settlement and the possible consequences for population density.

Return of the young

After dispersing for their first winter, many young shelducks return to their natal area, mostly when about one year old. Of 60 Ythan-reared ducklings (ringed before fledging) which returned, 45 per cent were first seen as yearlings, 40 per cent as two-year-olds and 15 per cent at three years old (Patterson et al., in press b).